GB578274A - Improvements relating to signal transmission systems - Google Patents

Abstract

578,274. Pulse modulation. FERRANTI, Ltd., and TAYLOR, M. K. June 3, 1943, Nos. 8887/43 and 8304/44. [Class 40 (v)] In a pulse signalling system, a sequence of spaced reference pulses is utilized and between each successive pair of such pulses is interposed a pulse, the timing of which varies with the momentary value of the modulating voltage or signal. The transmitter, Fig. 1, may comprise a back-coupled oscillation generator 1 utilizing a phasing network 2, the output being fed by potential-dividers 5, 4 to circuits generating the reference pulses and the modulated pulses respectively. The divider 5 is connected, through a network 7 ... 10 giving a phase lag of 90 degrees, to the grid of an amplifier, the output transformer 19 feeding a double-diode rectifier 22, 23 and giving a pulse output of doubled frequency across the load resistance 24. The pulses are passed to the grid of a limiter valve 26, producing narrow voltage pulses at its anode, which constitute the reference pulses referred to. For the production of the modulated pulses, the divider 4 is connected to the inner grids of valves 36, 39 through networks 34, 40 and 37, 41 which are arranged respectively to advance and to retard the applied voltage by 45 degrees. The outer control grids of these valves are connected in push-pull to the source 43 of modulating signal, and the anodes are connected to each other by resistances 44, 45 at the junction of which the phase displacement varies with the momentary value of the signal voltage. This phasedisplaced voltage is amplified in valve 48, doubled in frequency by rectifiers 53, 54, the rounded pulses thus produced being reduced to narrow pulses in the limiter valve 57, the output of which is added to that of the valve 26 in the other channel. The combined pulse sequence is amplified in the valves 28, 30, the latter being arranged as a cathode follower feeding a radio transmitter or a cable. Instead of using the valve pair 36, 39 as modulator, the unmodulated pulses may be passed through a delay line, of which the cores of the inductances have their permeability varied by the modulating signal. At the receiving station, Fig. 2, the pulses are applied to the grid of a back-biassed valve 58 for removing noise in the pulse gaps, and the anode pulses are applied as negative voltages to the grids of a pair of reciprocally-coupled valves 64, 67. Each pulse reverses the conductive condition of the valves until the next pulse restores the original condition. The output of the valve 67 is of square wave form and is fed via smoothing means to a loudspeaker or other instrument. For facsimile telegraphy, one set of pulses may be of longer duration than the other set, and the timeconstants of the circuits of the valves 64, 67 dissimilar, so that one valve is only rendered conductive by the longer pulses. The respective sets of pulses may be radiated on the same or on different carrier frequencies. The reference pulses need not be evenly spaced provided that the ratio of the time interval between reference pulses to the time interval between a reference pulse and the succeeding modulated pulse is inversely proportional to the momentary value of the modulating signal.